Patent GB-1367037-A (GB)
Title: Monitoring material
IPC: G21C17/10
Filed: 1972-07-10
Abstract: 1367037 producing neutrons; monitoring reactors; detecting neutrons UNITED STATES ATOMIC ENERGY COMMISSION 10 July 1972 [5 Aug 1971] 32167/72 Headings G6C and G6P [Also in Division G1] A device for monitoring fissile material comprises an isotopic neutron source, a neutron detector and means for causing the source to oscillate between positions respectively proximate and remote from the detector. The device is applicable to monitoring the reactivity and sub-criticality of nuclear fuel in storage or in a sub-critical nuclear reactor, or of radio-active wastes in storage. In one embodiment (Fig. 1) the neutron detector 15 is placed adjacent a nuclear fuel element 11 in storage, although it can be placed adjacent a neighbouring element. A motor 10 is also mounted adjacent fuel element 11, although in a complex fuel element array, the detector may be mounted near the array centre or periphery while the motor is mounted at the periphery in a position opposite the motor. The motor 10 comprises an hydraulic cylinder 17 in which a shuttle 27 is movable between a position 19 proximate the detector and a position 21 remote from the detector. The shuttle 27 encloses an isotopic neutron source 31. An hydrogeneous liquid 29 is admitted alternately to ports 23, 25 of the cylinder 17 to move the shuttle 27 to and fro, and, in the remote position 21 of the shuttle, shields the element 11 and detector 15 from neutrons from the source 31. Admission of liquid 29 to the cylinder is by way of a solenoid valve 37 controlled by a timer 53 which enables and alternately disables respective sealers 55, 57 such that the former receives a prompt neutron count when shuttle 27 is in proximate position 19, while the latter receives a delayed neutron count when shuttle 27 is in remote position 21. The reactivity of the fuel elements is then determined by a data-processing unit 61. In another embodiment (Fig. 4) the neutron detector 101 is disposed adjacent a fuel element 103 in a near-critical system such as a reactor being brought to criticality. A motor 91 drives a member 93 which mounts at one end an encapsulated neutron source 95 and at the other end a mass 97. A wave-form transmitter 99 senses the rotation of member 93 and generates the corresponding sinusoidal waveform (Fig. 5, not shown); alternatively, such waveform can be derived directly from the motor windings. A waveform comparator 105 compares this waveform with the (similar) detector output and by noting the phase-lag of the neutron response, the reactivity can be determined. The rotary neutron source arrangement can be replaced by a reciprocating linear arrangement. In a further embodiment (Fig. 3, not shown) applied to the monitoring of radio-active waste (71) in a storage tank (65) the detector (79) is mounted in a block (75) of gamma-shielding material which also supports an hydraulic motor (81) including a reciprocable shuttle (83). The block can be moved to different vertical locations within the tank to enable monitoring of the different settlement levels of material in the tank. Particular neutron sources mentioned for use in these embodiments comprise californium-252 or plutonium-beryllium or antimony-beryllium. In the latter instance, an antimony shuttle could be reciprocated into and out of a beryllium sleeve.